Thanks for the welcome, I should have been more clear. I received the companion ticket as part of a US promotion for flying on BA earlier this year. The terms state:

"Book your ticket and your companion ticket before July 30 and travel through December 15, 2010, with all travel completed by December 31, 2010."

I take it I should be okay even if I change the ticket dates (as long as I travel before 12/31), but I just wanted to confirm.